The objective of this project is to develop and apply methods for measurement of the volumes of the pulmonary circulation in conscious humans, anesthetized humans and animals. These volumes include total pulmonary blood volume, pulmonary arterial blood volume, capillary blood volume and pulmonary tissue volume. In addition, because some of these methods depend upon the distribution of inspired gas, investigation of the frequency dependency of nitrogen washout and its relationships to frequency dependency of lung compliance has also been part of the investigative effort. In this current year we have shown that the nitrous oxide pulmonary capillary blood flow measurement correlates closely with dye dilution estimates of cardiac output in anesthetized dogs. The correlation improves when right to left shunt as determined by 100 percent oxygen breathing is taken into account. We have also developed techniques to measure pulmonary arterial circulation time by the ether plethysmographic method and by a new pneumotachographic method. We have written programs for the small digital computer in order to analyze these data on-line. We have developed a rebreathing technique for measurement of pulmonary capillary blood volume and flow, and pulmonary tissue volume both in humans and in animals. This technique promises to be an advance in non-invasive estimates of pulmonary circulatory abilities. We have applied the nitrous oxide plethysmographic technique in human volunteers and the effects of postural drainage and exercise, and the effects of pharmacologic agents. We are currently using this method to indirectly assess pulmonary arterial pressure, but as yet we have not yet validated the accuracy of this test. We have studied the effects of lung volume and intrapleural pressure on pulmonary arterial blood volume, and have been able to dissect out influence of lung volume from intrapleural pressure volume with suitable respiratory maneuvers.